Document the overlay filesystem.
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| 1 | +Written by: Neil Brown <neilb@suse.de> | |
| 2 | + | |
| 3 | +Overlay Filesystem | |
| 4 | +================== | |
| 5 | + | |
| 6 | +This document describes a prototype for a new approach to providing | |
| 7 | +overlay-filesystem functionality in Linux (sometimes referred to as | |
| 8 | +union-filesystems). An overlay-filesystem tries to present a | |
| 9 | +filesystem which is the result over overlaying one filesystem on top | |
| 10 | +of the other. | |
| 11 | + | |
| 12 | +The result will inevitably fail to look exactly like a normal | |
| 13 | +filesystem for various technical reasons. The expectation is that | |
| 14 | +many use cases will be able to ignore these differences. | |
| 15 | + | |
| 16 | +This approach is 'hybrid' because the objects that appear in the | |
| 17 | +filesystem do not all appear to belong to that filesystem. In many | |
| 18 | +cases an object accessed in the union will be indistinguishable | |
| 19 | +from accessing the corresponding object from the original filesystem. | |
| 20 | +This is most obvious from the 'st_dev' field returned by stat(2). | |
| 21 | + | |
| 22 | +While directories will report an st_dev from the overlay-filesystem, | |
| 23 | +all non-directory objects will report an st_dev from the lower or | |
| 24 | +upper filesystem that is providing the object. Similarly st_ino will | |
| 25 | +only be unique when combined with st_dev, and both of these can change | |
| 26 | +over the lifetime of a non-directory object. Many applications and | |
| 27 | +tools ignore these values and will not be affected. | |
| 28 | + | |
| 29 | +Upper and Lower | |
| 30 | +--------------- | |
| 31 | + | |
| 32 | +An overlay filesystem combines two filesystems - an 'upper' filesystem | |
| 33 | +and a 'lower' filesystem. When a name exists in both filesystems, the | |
| 34 | +object in the 'upper' filesystem is visible while the object in the | |
| 35 | +'lower' filesystem is either hidden or, in the case of directories, | |
| 36 | +merged with the 'upper' object. | |
| 37 | + | |
| 38 | +It would be more correct to refer to an upper and lower 'directory | |
| 39 | +tree' rather than 'filesystem' as it is quite possible for both | |
| 40 | +directory trees to be in the same filesystem and there is no | |
| 41 | +requirement that the root of a filesystem be given for either upper or | |
| 42 | +lower. | |
| 43 | + | |
| 44 | +The lower filesystem can be any filesystem supported by Linux and does | |
| 45 | +not need to be writable. The lower filesystem can even be another | |
| 46 | +overlayfs. The upper filesystem will normally be writable and if it | |
| 47 | +is it must support the creation of trusted.* extended attributes, and | |
| 48 | +must provide valid d_type in readdir responses, so NFS is not suitable. | |
| 49 | + | |
| 50 | +A read-only overlay of two read-only filesystems may use any | |
| 51 | +filesystem type. | |
| 52 | + | |
| 53 | +Directories | |
| 54 | +----------- | |
| 55 | + | |
| 56 | +Overlaying mainly involves directories. If a given name appears in both | |
| 57 | +upper and lower filesystems and refers to a non-directory in either, | |
| 58 | +then the lower object is hidden - the name refers only to the upper | |
| 59 | +object. | |
| 60 | + | |
| 61 | +Where both upper and lower objects are directories, a merged directory | |
| 62 | +is formed. | |
| 63 | + | |
| 64 | +At mount time, the two directories given as mount options "lowerdir" and | |
| 65 | +"upperdir" are combined into a merged directory: | |
| 66 | + | |
| 67 | + mount -t overlayfs overlayfs -olowerdir=/lower,upperdir=/upper,\ | |
| 68 | +workdir=/work /merged | |
| 69 | + | |
| 70 | +The "workdir" needs to be an empty directory on the same filesystem | |
| 71 | +as upperdir. | |
| 72 | + | |
| 73 | +Then whenever a lookup is requested in such a merged directory, the | |
| 74 | +lookup is performed in each actual directory and the combined result | |
| 75 | +is cached in the dentry belonging to the overlay filesystem. If both | |
| 76 | +actual lookups find directories, both are stored and a merged | |
| 77 | +directory is created, otherwise only one is stored: the upper if it | |
| 78 | +exists, else the lower. | |
| 79 | + | |
| 80 | +Only the lists of names from directories are merged. Other content | |
| 81 | +such as metadata and extended attributes are reported for the upper | |
| 82 | +directory only. These attributes of the lower directory are hidden. | |
| 83 | + | |
| 84 | +whiteouts and opaque directories | |
| 85 | +-------------------------------- | |
| 86 | + | |
| 87 | +In order to support rm and rmdir without changing the lower | |
| 88 | +filesystem, an overlay filesystem needs to record in the upper filesystem | |
| 89 | +that files have been removed. This is done using whiteouts and opaque | |
| 90 | +directories (non-directories are always opaque). | |
| 91 | + | |
| 92 | +A whiteout is created as a character device with 0/0 device number. | |
| 93 | +When a whiteout is found in the upper level of a merged directory, any | |
| 94 | +matching name in the lower level is ignored, and the whiteout itself | |
| 95 | +is also hidden. | |
| 96 | + | |
| 97 | +A directory is made opaque by setting the xattr "trusted.overlay.opaque" | |
| 98 | +to "y". Where the upper filesystem contains an opaque directory, any | |
| 99 | +directory in the lower filesystem with the same name is ignored. | |
| 100 | + | |
| 101 | +readdir | |
| 102 | +------- | |
| 103 | + | |
| 104 | +When a 'readdir' request is made on a merged directory, the upper and | |
| 105 | +lower directories are each read and the name lists merged in the | |
| 106 | +obvious way (upper is read first, then lower - entries that already | |
| 107 | +exist are not re-added). This merged name list is cached in the | |
| 108 | +'struct file' and so remains as long as the file is kept open. If the | |
| 109 | +directory is opened and read by two processes at the same time, they | |
| 110 | +will each have separate caches. A seekdir to the start of the | |
| 111 | +directory (offset 0) followed by a readdir will cause the cache to be | |
| 112 | +discarded and rebuilt. | |
| 113 | + | |
| 114 | +This means that changes to the merged directory do not appear while a | |
| 115 | +directory is being read. This is unlikely to be noticed by many | |
| 116 | +programs. | |
| 117 | + | |
| 118 | +seek offsets are assigned sequentially when the directories are read. | |
| 119 | +Thus if | |
| 120 | + - read part of a directory | |
| 121 | + - remember an offset, and close the directory | |
| 122 | + - re-open the directory some time later | |
| 123 | + - seek to the remembered offset | |
| 124 | + | |
| 125 | +there may be little correlation between the old and new locations in | |
| 126 | +the list of filenames, particularly if anything has changed in the | |
| 127 | +directory. | |
| 128 | + | |
| 129 | +Readdir on directories that are not merged is simply handled by the | |
| 130 | +underlying directory (upper or lower). | |
| 131 | + | |
| 132 | + | |
| 133 | +Non-directories | |
| 134 | +--------------- | |
| 135 | + | |
| 136 | +Objects that are not directories (files, symlinks, device-special | |
| 137 | +files etc.) are presented either from the upper or lower filesystem as | |
| 138 | +appropriate. When a file in the lower filesystem is accessed in a way | |
| 139 | +the requires write-access, such as opening for write access, changing | |
| 140 | +some metadata etc., the file is first copied from the lower filesystem | |
| 141 | +to the upper filesystem (copy_up). Note that creating a hard-link | |
| 142 | +also requires copy_up, though of course creation of a symlink does | |
| 143 | +not. | |
| 144 | + | |
| 145 | +The copy_up may turn out to be unnecessary, for example if the file is | |
| 146 | +opened for read-write but the data is not modified. | |
| 147 | + | |
| 148 | +The copy_up process first makes sure that the containing directory | |
| 149 | +exists in the upper filesystem - creating it and any parents as | |
| 150 | +necessary. It then creates the object with the same metadata (owner, | |
| 151 | +mode, mtime, symlink-target etc.) and then if the object is a file, the | |
| 152 | +data is copied from the lower to the upper filesystem. Finally any | |
| 153 | +extended attributes are copied up. | |
| 154 | + | |
| 155 | +Once the copy_up is complete, the overlay filesystem simply | |
| 156 | +provides direct access to the newly created file in the upper | |
| 157 | +filesystem - future operations on the file are barely noticed by the | |
| 158 | +overlay filesystem (though an operation on the name of the file such as | |
| 159 | +rename or unlink will of course be noticed and handled). | |
| 160 | + | |
| 161 | + | |
| 162 | +Non-standard behavior | |
| 163 | +--------------------- | |
| 164 | + | |
| 165 | +The copy_up operation essentially creates a new, identical file and | |
| 166 | +moves it over to the old name. The new file may be on a different | |
| 167 | +filesystem, so both st_dev and st_ino of the file may change. | |
| 168 | + | |
| 169 | +Any open files referring to this inode will access the old data and | |
| 170 | +metadata. Similarly any file locks obtained before copy_up will not | |
| 171 | +apply to the copied up file. | |
| 172 | + | |
| 173 | +On a file opened with O_RDONLY fchmod(2), fchown(2), futimesat(2) and | |
| 174 | +fsetxattr(2) will fail with EROFS. | |
| 175 | + | |
| 176 | +If a file with multiple hard links is copied up, then this will | |
| 177 | +"break" the link. Changes will not be propagated to other names | |
| 178 | +referring to the same inode. | |
| 179 | + | |
| 180 | +Symlinks in /proc/PID/ and /proc/PID/fd which point to a non-directory | |
| 181 | +object in overlayfs will not contain valid absolute paths, only | |
| 182 | +relative paths leading up to the filesystem's root. This will be | |
| 183 | +fixed in the future. | |
| 184 | + | |
| 185 | +Some operations are not atomic, for example a crash during copy_up or | |
| 186 | +rename will leave the filesystem in an inconsistent state. This will | |
| 187 | +be addressed in the future. | |
| 188 | + | |
| 189 | +Changes to underlying filesystems | |
| 190 | +--------------------------------- | |
| 191 | + | |
| 192 | +Offline changes, when the overlay is not mounted, are allowed to either | |
| 193 | +the upper or the lower trees. | |
| 194 | + | |
| 195 | +Changes to the underlying filesystems while part of a mounted overlay | |
| 196 | +filesystem are not allowed. If the underlying filesystem is changed, | |
| 197 | +the behavior of the overlay is undefined, though it will not result in | |
| 198 | +a crash or deadlock. | |
| @@ -6832,6 +6832,13 @@ F: drivers/scsi/osd/ · 6831 unmodified lines | ||
| 6832 | 6832 | F: include/scsi/osd_* |
| 6833 | 6833 | F: fs/exofs/ |
| 6834 | 6834 | |
| 6835 | +OVERLAYFS FILESYSTEM | |
| 6836 | +M: Miklos Szeredi <miklos@szeredi.hu> | |
| 6837 | +L: linux-fsdevel@vger.kernel.org | |
| 6838 | +S: Supported | |
| 6839 | +F: fs/overlayfs/* | |
| 6840 | +F: Documentation/filesystems/overlayfs.txt | |
| 6841 | + | |
| 6835 | 6842 | P54 WIRELESS DRIVER |
| 6836 | 6843 | M: Christian Lamparter <chunkeey@googlemail.com> |
| 6837 | 6844 | L: linux-wireless@vger.kernel.org |